1. Field of the Invention
The invention relates to a method of estimating an irradiation dose imparted to patients receiving radiation treatment. Specifically, the invention relates a method of estimating an irradiation dose based on absorption measurements of a radiochromic film at two absorption wavelengths.
2. Description of the Related Art
Radiochromic films are increasingly used in medical dosimetry in assessing radiation doses imparted to patients receiving radiation treatment. Radiochromic films are transparent films which change from colorless to different shades of a bluish color upon irradiated by ionizing radiation. The degree of shading depends on the amount of energy deposited on the films. They are insensitive to daylight and are tissue equivalent; their responses are slightly energy dependent and practically dose-rate independent. They can offer a very fine resolution up to 1200 lines/mm due to their grainless nature. All these advantages render them a better two-dimensional dosimetry medium over the silver-halide type of radiographic film. Radiochromic films have been therefore found to be a valuable tool in high-resolution film dosimetry where steep dose gradients are encountered.
An exposed radiochromic film needs to be read out before any quantitative analysis of absorbed doses can be made.
With devices such as the microdensitometer disclosed in U.S. Pat. No. 6,927,859, which allows high-resolution read out of radiochromic films at any suitable wavelengths, it is now possible to obtain absorption data at any given wavelengths. It is known that the absorption spectrum of the radiochromic films has two distinct absorption peaks in the visible spectrum, the major peak at about 672 nm and the minor peak at about 610 nm. On the basis of this spectral information, one may realize that the measurement of the transmittance should be made at the major absorption peak to attain the maximum sensitivity of the dose response. However, the measurement may easily saturate at high doses due to the strong absorption at these levels. On the other hand, if the measurement is carried out at the wavelength of the minor peak, the measurement will not be sensitive enough to detect low doses. Thus, an estimation method relying on a single wavelength absorption data has its shortcomings.